The present invention relates to a sheet type electrode for use in the measurement of ions, such as an electrode used in the measurement of an ionic concentration such as pH. 2. Description of the Technical Background
Recently, in order to miniaturize a construction of an electrode for measuring an ionic concentration such as pH while reducing the cost of production, and improving its operation and maintenance, there have been attempts to provide the electrode in the form of a sheet.
FIG. 5 shows an external appearance of a sheet type composite electrode for use in the measurement of pH according to a recently filed Japanese Patent Application No. Sho 61-285371 filed by the present Applicant Horiba, Ltd. and FIG. 6 shows a cross section of a measuring electrode of FIG. 5. Referring now to both FIG. 5 and FIG. 6, reference numeral 51 designates a substrate formed of a material, such as a polyethylene terephthalate (hereinafter referred to as PET), having a sufficiently high electrical insulating property even when it is immersed in a solution containing electrolytes. Reference numeral 52 designates an electrode formed on an upper surface of the substrate 51. The electrode 52 can be formed by subjecting the substrate to a preparatory treatment, for example, an appointed surface pretreatment and then a silk screen printing of an Ag paste to form electrode 52. A portion of the electrode 52 is formed as an internal electrode portion 53 coated with an electrode material such as AgCl, and the rest of the electrode 52 is formed as an electrical lead portion 54.
A support layer 55 is formed over a portion of the substrate 51 and is provided with a through hole 56 at a place corresponding to the internal electrode portion 53. The support layer 55 is formed of a material, such as PET, having a sufficiently high electrical insulating property even though it is to be immersed in solutions containing electrolytes. The support layer 55 can be formed on the substrate 51 while still exposing the lead portion 54 and a circumference area around the lead portion 54.
The through hole 56 can be filled with a disc type gelatinized internal solution 57 prepared by adding a gelatinizer, such as agar-agar, and a gel-evaporation inhibitor, such as glycerine, to a basic internal solution obtained by adding a phosphoric acid-buffering agent to a 3.3M-aqueous solution of KC1 supersaturated with for example AgC1. The mixture can be heated to turn it into a paste and then placed on the internal electrode portion 53 by a screen printing method and the like, so that an upper surface of the disc type gelatinized internal solution 57 may be slightly projected over an upper surface of the support layer 55.
Reference numeral 58 designates an ion-selective response membrane formed in a thin film-like shape. The ion-selective response membrane 58 is obtained for example by dissolving polyvinyl chloride resin powders in tetrahydrofuran as a solvent, adding tri-n-octylphosphine oxide as a hydrogen ion-responsive substance to the resulting solution together with a plasicizer, while sufficiently stirring the resulting mixture, and evaporating tetrahydrofuran. The ion-selective response membrane 58 is fixedly mounted on the upper surface of the support layer 55 along a circumference of the hole 56 by the use of organic high molecular adhesives 59, such as a polyvinyl chloride resin series adhesive or silicon series adhesive, also having a sufficiently high electrical insulating property. The membrane 58 forms an interface of the measuring electrode with its lower surface adhered to an upper surface of the gelatinized internal solution 57 which is tightly filled on the inside of the through hole 56.
Referring to FIG. 5, reference numeral 60 designates a liquid junction membrane that can be formed of an inorganic sintered porous material, an organic high molecular porous material or the like impregnated with KC1. The liquid junction membrane 60 is also fixedly mounted on the upper surface of the support layer 55 along a circumference thereof so that its lower surface may contact an upper surface of another gelatinized internal solution (not shown) to be formed as a reference electrode. The internal construction of the liquid junction membrane 60 can be almost the same as that of the measuring electrode shown in FIG. 6. Reference numeral 61 designates a casing whose upper surface forms a sample solution holder portion.
With the above described construction, adhesives capable of maintaining a complete seal between the support layer 55 and the response membrane 58 are difficult to obtain. Although polyvinyl chloride resin series organic high molecular adhesives are superior in providing a speedy drying property, they have a disadvantage in that a surface coating is apt to be produced. In addition, silicon series organic high molecular adhesives have problems in providing the stability of adhering conditions such as hardening time and viscosity. In short, the use of any one of these organic high molecular adhesives cannot be expected to provide a complete seal. Accordingly, an electrode having the above described construction has room for improvement in this respect.